Stability augmentation system for light aircraft providing pilot assist and turn

ABSTRACT

A small aircraft with a conventional stability augmentation system for automatically maintaining a wings level constant heading attitude is provided with a switch on the control wheel which is spring biased to a central neutral position and which is actuated by the pilot to the right or left of neutral and held in the actuated position to make a right or left turn. When actuated, the switch introduces an offset signal into the stability augmentation system causing the system to produce and stabilize on a turn rate determined by the magnitude of the offset signal. The actuated switch also sets up an additional signal of opposite sense to the offset signal which is substituted for the offset signal whenever the turn rate exceeds a preset maximum, as may occur if the pilot for any reason increases the turn rate above the stabilized rate by applying sufficient force to the wheel to overpower the stability augmentation system. The opposing signal causes the stability augmentation system to warn the pilot of the excessive rate by exerting a powerful backward force on the ailerons, and on the control wheel in opposition to the pilot&#39;&#39;s force, tending to reduce the bank angle and thereby the turn rate below the preset maximum. When the switch is released to its neutral position the stability augmentation system returns the aircraft to a wings level attitude.

United States Patent 1 1 3,730,461 Knemeyer 5] May 1, 1973 STABILITYAUGMENTATION SYSTEM [57] ABSTRACT FOR LIGHT AIRCRAFT PROVIDING A smallaircraft with a conventional stability augmen- PILOT ASSIST AND TURNtation system for automatically maintaining a wings [76] Inventor; Si fi d Knemeyer 4 0 Meadow level constant heading attitude is provided witha L p B 123 Yellow switch on the control wheel which is spring biased toa Springs, Ohio 45387 central neutral position and which is actuated bythe 7 pilot to the ri ht or left of neutral and held in the actu- [22]plied: May 1971 ated position to make a right or left turn. When actu-[21] Appl. No.: 140,403 ated, the switch introduces an offset signalinto the stability augmentation system causing thesystem to produce andstabilize on a turn rate determined by the [5.2] Cl. "244/77 244/77magnitude of the offset signal. The actuated switch 51 Int. Cl. ..B64c13/18, B64c 13 44 sets t nal S gnalof opposite sense to [58] Field atSearchm A, 77 R, 77 D, the offset signal which is substituted for theoffset 244/77 E, 77 F 77 V 77 SE, 79 83 R, 83 D signal whenever the turnrate exceeds a preset max- 83 E, 78; 318/489 566 585 imum, as may occurif the pilot for any reason increases the turn rate above the stabilizedrate by ap- 5 References Cited plying sufficient force to the wheel tooverpower the stability augmentation system. The opposing signal UNITEDSTATES PATENTS 1 causes the stability augmentation system to warn the@pilot of the excessive rate by exerting a powerful ibackward force onthe ailerons, and on the control 3,082,978 3/1963 Smith etal.........244 77 SE :Wheel in Opposition to the p force, tending 3,070,33212/1962 Hess, Jr. ..244/77 D ;reduce the bank angle and thereby the turnrate below 3,128,968 ,4/1964- Kaufma et a SE Zthe preset maximum. Whenthe switch is released to 3,l7l,6l5 3/1965 Patterson ..'....244/77 SEFOREIGN PATENTS OR APPLICATIONS Great Britain ..244/77 E tif its neutralposition the stability augmentation system greturns the aircraft to awings level attitude.

6 Claims, 3 Drawing Figures STABILITY AUGMENTATION SYSTEM FOR LIGHTAIRCRAFT PROVIDING PILOT ASSIST AND TURN There is reserved to theGovernment of the United States a nonexclusive, irrevocable,royalty-free license in the invention described herein with power togrant licenses for all governmental purposes.

BACKGROUND OF THE INVENTION The invention relates to aircraft flightcontrol systems, particularly control systems for small aircraft.

Small aircraft are frequently flown by pilots that are not proficient inflying by instruments. Such pilots may at times unavoidably encounterinstrument or [PR flight conditions and become disoriented due to lossof the visual ground reference.'When disoriented the pilot mayunknowingly place the aircraft in dangerous flight attitudes anddynamics, such as excessive bank angles and turn rates, which may causethe aircraft to crash. Small aircraft are often equipped with relativelysimple stability augmentation systems which operate to automaticallyhold the aircraft in a wings level attitude. Such systems howeverincrease the difficulty in executing intentional turns since the pilotmust exert suffi cient force on the control wheel to overpower. thestability augmentation system. To overcome this difficulty such systemsusually provide an ON-OFF switch on the control wheel so-that the systemmay be tuned OFF when executing intentional turns. While stabilityaugmentation systems of this type afford some protection under IFRconditions when ON, the disoriented pilot in attempting to do what hissenses erroneously tell him is the right thing may overpower the systemand force the aircraft into a dangerous attitude from which the systemmay not be able to recover.

SUMMARY OF THE INVENTION It is the purpose of the invention to providean improved stability augmentation system for small aircraft which maybe left ON at all times, which assists rather than opposes the pilot inexecuting intentional turns, which stabilizes the aircraft at apreselected safe turn rate in the same manner that the aircraft isnormally stabilized in a wings-level attitude, and which warns the pilotagainst excessive turn rates by strongly resisting any attempt to 1increase the turn rate above a preselected maximum value throughoverpowering the stability augmentation system at the control wheel.

The usual stability augmentation system for holding the aircraft in awings level attitude comprises basically a rate gyro, capable ofresponding to both roll rates and yaw rates of the aircraft to producean-error signal containing rate magnitude and direction information, anda servomechanism coupled to the ailerons and receiving the error signalas an input for actuating the ailerons in response to the error signalin such manner as to reduce the error signal to zero. In accordance withthe invention, a switch is provided on the control wheel of the aircraftwhich is spring loaded to a central neutral position and which the pilotactuates to the right or left and holds in the actuated position toexecute a right or left turn. The force required to actuate the controlsurfaces is supplied by the stability augmentation system which alsoacts during the turn to stabilize the aircraft at a preselected turnrate. Upon release of the switch to its neutral position, the stabilityaugmentation system returns the aircraft to a wings level attitude inthe usual manner. This is accomplished as follows:

When actuated, the switch algebraically adds to the gyro error signal atthe input to the servomechanism a signal which is of the same characteras the gyro error signal, which is preset in magnitude to a desired turnrate, and which has directional information corresponding to that of theerror signal that would be produced by the gyro if the aircraft were ina turn of opposite direction to that called for by the switch. Thissignal in effect offsets the zero point of the stability augmentationsystem which, in operating to reduce the servomechanism input to zero,is obliged to place the aircraft in a turn at such rate that the gyroerror signal just cancels the offset signal. The system therefore actsto stabilize the aircraft at a turn rate that depends upon the magnitudeof the offset signal which can be preset into the system.

Actuation of the switch also sets up an opposing signal of the samecharacter as the offset signal but with opposite directionalinformation. The gyro error signal is continuously compared with apreset reference signal representing a maximum permissible rate of turn.If the pilot for any reason increases the rate of turn above the rate atwhich the aircraft is stabilized by applying enough force to the controlwheel to overpower the stability augmentation system and this is carriedto the point where the gyro error signal exceeds the reference signal,the offset signal is removed and the opposing signal substitutedtherefor. The opposing signal adds to the gyro error signal to produce alarge input to the servomechanism which in response acts to exert astrong backward force on the control wheel against the pilots efforts.Because of the large input signal to the servomechanism, the backwardforce exerted in this circumstance is much greater than the normalresistance offered by the stability augmentation system to displacementfrom the stabilized turn rate and experienced by the pilot before thepreset maximum rate is reached. It therefore provides an unmistakablewarning to the'pilot that the maximum rate has been exceeded. If thepilot heeds the warning and ceases his effort at the control wheel, or,in any event, if the pilot force at the wheel is insufficient to resistthe servomechanism, the servomechanism automatically drives the aileronsand the control wheel backward to the other side of neutral thusreducing the bank angle and the turn rate until the rate has fallenbelow the preset maximum value. By this process the pilot is preventedfrom unintentionally forcing dangerouslyhigh turn rates on the aircraft.

7 BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THEINVENTION Referring to FIG. 1 of the drawing, 1 represents the controlwheel of a typical light aircraft coupled to the ailerons through asuitable mechanical linkage 2 for controlling the aircraft about itsroll axis. The rectangle 3 encloses the elements in schematic block formof a typical stability augmentation system as commonly used in suchaircraft for automatically maintaining straight and level flight.Stability augmentation systems are realized in various ways, i.e., asA.C., D.C., hydraulic, or vacuum powered systems. All perform the samefunction and all are adaptable to the invention. For illustrativepurposes, the system shown and to be described is a DC. system.

The system comprises a rate gyro 4 having a DC. pickoff or follow-uppotentiometer 5. Standard practice is to mount the rate gyro with itsaxis at an angle to the roll or longitudinal axis of the aircraft andparallel to a plane normal to the pitch axis so that both roll and yawrates are sensed. When the aircraft has no movement about either theroll or the yaw axis the contact 6 is at the center of the potentiometer5 and the gyro output signal is zero. When the aircraft rotates aboutits roll or yaw axis the contact 6 is displaced from center by an amountproportional to the rotational rate or angular velocity and in adirection determined by the direction of the rotation. The invention isapplied to the conventional stability augmentation system by breakingthe circuit between the pickoff 5 and the input to servoamplifier 7 at8-9, as will be explained later. In the conventional system this circuitis closed and the pickoff signal is applied directly to the amplifier.The servoamplifier provides the power required to drive servomotor 10which is coupled through clutch 11 to the aileron control linkage 2. Theservomotor 10 is shown as of the fixed field D.C. type with the fieldreceiving a constant energization from power source 12 by way ofconnection 13. The DC. output of the servoamplifier energizes thearmature of motor 10 by way of connection 14, causing the motor to runin a direction determined by the polarity of the amplifier output whichalways corresponds to the polarity of its input from pickoff 56. Inaddition to the field of motor 10, servoamplifier 7 and clutch 11 arealso energized from DC power source 12 through the ON-OFF switch 15usually provided with stability augmentation systems.

When the above described system is ON it operates to oppose any rotationof the aircraft about either its roll axis or its yaw axis, thus tendingto maintain the aircraft in straight and level flight. If, for example,a wind gust disturbs the straight and level flight of the aircraft, therate gyro produces an error signal which acts through the servoamplifier7, servomotor l0, and clutch 11 to displace the ailerons from theirneutral position in the proper direction to counteract the disturbanceand reduce the error signal to zero. With the system ON, intentionalturns by the pilot are similarly opposed and can only be accomplished byapplying sufficient force to the wheel 1 to overpower the stabilityaugmentation system. Therefore, a switch such as 15 is usually providedto disable the system during intentional maneuvers. When switch 15 isOFF, clutch 11 is released, thus decoupling the stability augmentationsystem from the aileron linkage and permitting the pilot to control theailerons with normal pressure on the control wheel 1.

Application of the invention to the above described system provides,with the system ON continuously, pilot assist and turn stabilization inintentional turns and automatic warning in the form of a strong opposingforce on the control wheel when the turn rate for any cause exceeds apreset value. In accordance with the invention a switch 16 is providedon thecontrol wheel 1 at a location convenient to the pilot. In theembodiment shown it is located at the top of the left horn of the wheeland is designed to be actuated by the left thumb. The switch is springbiased to a central neutral position. To make a right or left turn theswitch is actuated to the right or left and held in the actuatedposition for as long as it is desired for theturn to continue. Uponrelease of the switch the aircraft returns to straight and level flightunder control of the stability augmentation system. The electricaldetails of switch 16 are shown schematically within the rectangle alsodesignated 16.

The signals produced by switch 16 are coupled into the stabilityaugmentation system 3 through summing network 17, to one input 18 ofwhich they are applied through the contacts of relay 19. The other input20 is connected to terminal 8 and the output 21 is connected to terminal9, so that the circuit continuity between these terminals isreestablished through the summing network. The summing network may beany suitable network capable of producing a signal in output circuit 21equal to the algebraic sum of the signals in input circuits 18 and 20.The coil of relay 19 receives its energization from either differentialamplifier 22 or differential amplifier 23 to the outputs of which it isconnected through isolating diodes 24 and 25. Amplifiers 22 and 23 arealike except for the polarity of. their inputs. Amplifier 22 is designedto operate with positive input signals. Thus, whenever a positive inputsignal on input 26 exceeds a preset positive reference voltage on input27, derived from potentiometer28, relay 19 is actuated. Similarly,amplifier 23 is designed to operate with negative input signals andproduces an output to the coil of relay 19 whenever the absolutemagnitude of a negative signal on input 29 exceeds a preset negativereference voltage on input 30, derived from potentiometer 31. The outputsignal of. the rate gyro pickoff 5, available at terminal 8, is appliedto the inputs 26 and 29 of amplifiers 22 and 23 in parallel forcontrolling-the operation of relay 19.

The operation of the complete system as described above is as follows:First it will be assumed that the construction of the rate gyro and itspickoff 5 is such that a roll or yaw rate of the aircraft to the leftproduces a positive error signal at terminal 8 and a roll or yaw rate tothe right produces a negative error signal at this point. It followsthen that a positive error signal at the input of servoamplifier 7displaces the ailerons in the direction required to roll the aircraft tothe right and thus oppose a left roll or yaw. Similarly, a negativeerror signal at the servoamplifier provides the proper direction ofaileron displacement to roll the aircraft to the left and thus oppose aright roll or yaw. With switch 16 in its normal central position, thereis no signal on input 18 of summing network 17 and the aircraft ismaintained in straight and level flight by the stability augmentationsystem in conventional manner, the

summing network in this circumstance merely serving to complete thecircuit between terminals 8 and 9.

Assume now that the pilot wishes to execute a right turn. Switch 16 isactuated to the right and held in the actuated position. Positivevoltage is now applied from contact 60 of potentiometer 32 throughclosed contacts 33-34 of switch 16 and normally closed contacts 35-36 ofrelay 19 to input 18 of summing network 17, and thence through thisnetwork to the input terminal 9 of the aileron servomechanism. Since theaircraft is in straight and level flight there is no rate gyro errorsignal at terminal 8 and therefore the voltage from switch 16constitutes the entire input at terminal 9. The servomechanism respondsto this positive signal in the same manner in which it would respond toa positive error signal from the rate gyro produced by roll and yawrates to the left, i.e., it actuates the ailerons in the directionrequired to bank the aircraft to the right and thus initiates a rightturn.

The right roll and yaw rates which the aircraft is now undergoingproduce a negative error signal from the rate gyro which is appliedthrough terminal 8 to input of summing network 17 and to input 29 ofdifferential amplifier 23. The negative error signal reduces thepositive voltage at'terminal 9, however as long as this voltage ispositive the servomechanism continues to run in the initial directionand the bank angle, the rate of turn, and the error signal continue toincrease. When the increasing error signal has reduced the voltage atpoint' 9 to zero the servomechanism stops. However, since the aileronsare still displaced from their neutral position, the bank angle and rateof turn continue to increasecausing the error signal to exceed thepositive voltage at summing network input 18 and a negative voltage toappear'at terminal 9. This starts the servomechanism runningin thereverse direction, driving the ailerons to the other side of theirneutral position and as a result reducing the bank angle and turn rateuntil the negative error signal has become less than the positivevoltage at summing input 18. This produces a network voltage at terminal9 causing the servomechanism to again reverse its direction of rotation.By this process the stability augmentation system to input 29 ofdifferential amplifier-23 exceeds the negative reference voltage atinput 30, relay 19 is actuated and the negative voltage present atterminal37 in switch 16 is applied through contacts 38-39 of the switchand contacts -36 of relay 19 to the input 18 of the summing network. Atthe same time the offset voltage from contact 60 of the switch isremoved from summing network input 18. The negative voltage at contact37 adds to the negative error signal of the gyro in the summing network17 to produce a large negative signal at terminal 9. The servomechanismresponds to this large input signal by exerting a strong backward forceon the control wheel against the pilot's efforts. Because of the largeinput signal to the servomechanism, the backward force exerted in thiscase is much greater than the normal resistance offered by the stabilityaugmentation system to displacement from the stabilized turn rate andexperienced by the pilot before the preset maximum rate is reached. Ittherefore provides an unmistakable warning to the pilot that the maximumrate has been exceeded. If the pilot heeds the warning and ceases hiseffort at the control wheel, .01 reduces his effort to a forceinsufficient to resist the servomechanism, the servomechanismautomatically drives the ailerons and the control wheel backward to theother side of their neutral position thus reducing the turn rate untilthe negative gyro error signal has been reduced in magnitude to a valuesmaller than the negative reference voltage on input 30 of thedifferential amplifier. When this occurs relay 19 is deenergizedremoving the negative voltage from the summing network input 18 andrestoring the positive offset voltage derived from contact 60 ofpotentiometer 32. If the pilot has ceased his efforts to overpower thestability augmentation system, the system continues to reduce the turnrate until the gyro error signal has become equal to the offset voltageand again stabilizes the airstabilizes the turn rate of the aircraft atthe value for Y which the gyro error signal equals the voltage atcontact 60 of switch 16 and the net voltage at point 9 is zero. The turnrate at which the aircraft stabilizes is therefore determined by thevoltage at contact 60, which may be adjusted to provide any desired saferate, for example, 2/second. The voltage introduced into the stabilityaugmentation system by switch 16 in effect offsets the stability pointof the system so that the system stabilizes the aircraft at a certainbank angle and turn rate rather than at a wings level attitude or zeroturn rate. Considered in this way, the offset voltage for wings levelflight is zero, which is the voltage supplied by switch 16in its neutralposition.

Maximum turn rate limitation is provided by relay 19 under the controlof differential amplifiers 22 and 23. If the pilot should increase theright turn rate above the stabilized rate by applying sufficient forceto the control wheel to overpower the stability augmentation system andcarry this effort to the point where the negative error signal producedby the gyro and applied craft at the turn rate corresponding to thisvoltage. The system therefore prevents the pilot from unintentionallyexceeding the maximum safe turn rate preset at potentiometer 31. 1

Left turns-may be executed by actuating switch 16 to the left andholding it in the left position for the desired duration of the turn.The operation is the same as for a right turn except that differentialamplifier 22 and left maximum rate presetting potentiometer 28 areoperative in place of differential amplifier 23 and potentiometer 31.The right or left turn rate at which limitation occurs may be preset toany value at potentiometers 31 and 28. Usually this is the maximum safeturn rate for the particular aircraft, or a value near to this.

FIG. 2 shows a modification of the system of FIG. 1 in which a somewhatmore complex control wheel switch 16 is employed to provide, in additionto pilot assist and maximum turn rate limitation, turn ratestabilization at a plurality of preselected lower values (in this casetwo). To execute a turn at the lower of the two values the pilot movesthe switch lever 50 to the right until the first detent 51 is felt, andholds the switch at this position. This places contact 34 on contact 52and applies the positive voltage at adjustable contact 53 throughnormally closed contacts 35-36 of relay 19 to the input 18 of summingnetwork 17, and thence to the input of the servoamplifier at point 9.The positive voltage at point 9 initiates a right turn of the aircraftand the stability augmentation system stabilizes the turn rate at avalue determined by the voltage at contact 53 in the same manner asalready explained for FIG. 1. A

higher turn rate may be achieved by actuating the switch to the seconddetent position 54, in which case contact 34 rests on contact 55 and theturn rate is increased to and stabilized at a value determined by thevoltage at contact 56. Contacts 53 and 56 may be set, for example, tolow and moderate turn rates such as llsecond and 2lsecond. Any'actuationof the switch from its neutral position energizes contact 39 for maximumturn rate limitation in the manner explained for FIG. 1. The turn may beended by releasing switch actuator 50 and allowing it to return to itsneutral position under the influence of spring 57. The detent is madestrong enough to give a positive indication to the pilot but not enoughto resist the return spring.

FIG. 3 is a modification of switch 16 of FIG. 2 in which the voltage atcontact 34 increases continuously as the switch is displaced from itsneutral position, rather than in steps as in FIG. 2. In this case anydesired stabilized turn rate, up to the maximum set at potentiometer 31,can be achieved depending upon the I distance from its neutral positionat which the switch is held. If desired, detents may be used in thiscase also to mark standard rates. The use of detents may be madeselective by employing a detent spring retractor 58.

As in FIG. 1, the operation for left turns is the same as for rightturns except that the switch is actuated to the left and turn ratelimitation is provided by differential amplifier 22 rather than bydifferential amplifier 23.

Certain existing stability augmentation systems for small aircraftemploy vacuum actuators for the ailerons. Vacuum is admitted to theseactuators through a control valve that is mechanically coupled to therate gyro. In applying the invention to a system of this type, thevoltage at contact 36 of relay 19 is applied, through a suitablecoupling network, to a torquer on the control valve for controlling itsposition independently of the gyro. In this case, summing network 17 andall apparatus beyond in the drawing are eliminated. In general, inapplying the invention to a stability augmentation system of any type,the output of switch 16, as it appears at contact 36 of relay 19, isutilized directly or indirectly to provide an input to theservomechanism which controls the ailerons that is in addition to theinput supplied by the rate gyro.

I claim:

1. In an aircraft having a manual .roll axis control directly coupled tothe ailerons and having a stability augmentation system comprising arate gyro that.

responds to roll and yaw rates to producev an error would have if saidaircraft were turning in a direction opposite to the turn directioncorresponding to the direction of switch actuation, said switc alsooperating when actuated to produce an additional signal of the samecharacter as said offset signal but having opposite directionalinformation; means for algebraically adding said offset signal to saiderror signal at the input to said servomechanism; and means forcomparing the absolute magnitude of said error signal with the absolutemagnitude of a reference signal representing a predetermined maximumrate of turn and operative whenever said error signalis larger than saidreference signal to substitute said additional signal for said offsetsignal as the signal algebraically added to said error signal at theinput to said servomechanism.

2. Apparatus as claimed in claim 1 in which said switch is constructedto produce an offset signal that increases in a plurality of steps withthe degree ofactuation from the neutral position and to produce-"saidadditional signal for all degrees of actuation from the neutralposition.

3. Apparatus as claimed in claim 2 in which the offset signal 7 stepsare marked by switch detents strong 'enoughto be sensed by the pilot butnot strong enough to interfere with the return of the switch to itsneutral position when released.

4. Apparatus as claimed in claim 1 in which said switch is constructedto produce an offset signal that in-. creases continuously with thedegree of actuationfrom the neutral position and to produce saidadditional signal for all degrees of actuation from the neutralposition.

5. Apparatus as claimed in claim 4 in which at least one value of saidoffset signal .is'marked by a switch detent strong enough to be sensedby' the pilot but not strong enough to interfere with the return of theswitch to its neutral position when released.

6. Apparatus asclaimed in claim 1 in which the manual control is of thetwo-horn wheel type, in which I the switch is mounted at the top of oneof the horns, and in which the switch has an actuating lever with a U-shaped thumb receptacle to permit actuation to either side of theneutral position by the thumb of the pilot.

1. In an aircraft having a manual roll axis control directly coupled tothe ailerons and having a stability augmentation system comprising arate gyro that responds to roll and yaw rates to produce an error signalhaving rate magnitude and rate direction information and aservomechanism coupled to said ailerons and receiving said error signalat its input for automatically maintaining said aircraft in a wingslevel constant heading attitude: a switch spring biased to a neutralposition and actuatable by the pilot to the right or left of neutral toexecute a right or left turn, said switch operating when actuated toproduce an offset signal of the same character as said error signal andhaving the same directional information that said error signal wouldhave if said aircraft were turning in a direction opposite to the turndirection corresponding to the direction of switch actuation, saidswitch also operating when actuated to produce an additional signal ofthe same character as said offset signal but having opposite directionalinformation; means for algebraically adding said offset signal to saiderror signal at the input to said servomechanism; and means forcomparing the absolute magnitude of said error signal with the absolutemagnitude of a reference signal representing a predetermined maximumrate of turn and operative whenever said error signal is larger thansaid reference signal to substitute said additional signal for saidoffset signal as the signal algebraically added to said error signal atthe input to said servomechanism.
 2. Apparatus as claimed in claim 1 inwhich said switch is constructed to produce an offset signal thatincreases in a plurality of steps with the degree of actuation from theneutral position and to produce said additional signal for all degreesof actuation from the neutral position.
 3. Apparatus as claimed in claim2 in which the offset signal steps are marked by switch detents strongenough to be sensed by the pilot but not strong enough to interfere withthe return of the switch to its neutral position when released. 4.Apparatus as claimed in claim 1 in which said switch is constructed toproduce an offset signal that increases continuously with the degree ofactuation from the neutral position and to produce said additionalsignal for all degrees of actuation from the neutral position. 5.Apparatus as claimed in claim 4 in which at least one value of saidoffset signal is marked by a switch detent strong enough to be sensed bythe pilot but not strong enough to interfere with the return of theswitch to its neutral position when released.
 6. Apparatus as claimed inclaim 1 in which the manual control is of the two-horn wheel type, inwhich the switch is mounted at the top of one of the horns, and in whichthe switch has an actuating lever with a U-shaped thumb receptacle topermit actuatIon to either side of the neutral position by the thumb ofthe pilot.